House Republicans who last year made good on longstanding promises to overhaul the mental health system could roll back coverage for millions of people with mental illness and addiction problems by overhauling Medicaid as part of an Obamacare repeal package.

Legislation being marked up Wednesday would phase out Obamacare’s Medicaid expansion that covers 1.2 million Americans with serious mental illness and substance abuse problems, as well as scrap baseline coverage requirements. The change means certain beneficiaries would no longer get coverage for mental health and substance abuse treatments guaranteed under the ACA.

"We are really worried about its implications for people living with mental illness and addiction issues," said Chuck Ingoglia, senior vice president of public policy at the National Council for Behavioral Health.

The Medicaid overhaul comes after Republicans in the last Congress approved what Speaker Paul Ryan dubbed “the most significant reforms in a decade” to the mental health system. The bill authorized treatment and prevention programs and created a new assistant secretary for mental health at HHS.

Some of those efforts could be undercut by more sweeping Medicaid revisions, according to some lawmakers and advocacy groups.

The Medicaid aspects of Obamacare repeal are prompting concern from four Republican senators who are concerned about the prospect of reduced access to mental health and addiction services. Rob Portman of Ohio, Shelley Moore Capito of West Virginia, Cory Gardner of Colorado and Lisa Murkowski of Alaska signaled they could withhold support for the House plan in a letter to Senate Majority Leader Mitch McConnell, declaring “reform should not come at the cost of disruption in access to healthcare for our country’s most vulnerable and sickest individuals.”

Sen. Chris Murphy (D-Conn.), who co-authored last year's mental health overhaul package, called the House GOP bill a “dumpster fire that everyone should hate," and nearly all of the country’s largest mental health groups have lined up against the legislation in its current form.

“We strongly encourage policymakers to recognize that all decisions about health reform must consider and provide for the needs of the most severely mentally ill," a spokesperson for the Treatment Advocacy Center said in a statement. "Failure to do so will forfeit the momentum towards reforming our broken mental health system at tremendous human and financial cost."

The Obamacare repeal bill hasn’t been scored yet by CBO, meaning it’s unclear exactly how many people could lose coverage. But mental health advocates say vulnerable populations could fare badly.

The legislation would halt Obamacare's Medicaid expansion in 2020. Enrollees could stay on their plans, but anyone without continuous coverage for more than 30 days would lose eligibility. That worries advocates who say people with mental illness or addiction issues frequently drop off the rolls for periods of time.

Advocates are also worried about the way the bill would replace Medicaid's funding structure with capped payments to states based on the number of people enrolled in their Medicaid programs — a move likely to lead to federal spending cuts.

“States will be required to make hard choices on which benefits they want to continue and on enrollment numbers,” said Stuart Gordon, policy director for the National Association of State Mental Health Program Directors. “We would hope that mental health and substance use disorder benefits and coverage ... will not be eliminated under that scenario, but the decisions will be made on a state-by-state basis based on state budgets, and that very well could lead to significant variations in benefits and coverage by states.”

Chronic conditions like serious mental illness and substance use disorder are especially costly to treat. According to MACPAC, enrollees with a behavioral health diagnosis account for about $131 billion in spending, almost half of total Medicaid expenditures in 2011.

“I have no idea whether the per capita caps for the elderly and disabled category are sufficient to accommodate the clinical needs of somebody with schizophrenia or bipolar disorder,” a source in mental health policy said. “They are a high risk and expensive patient population with per capita spending exceeding $35,000 per year. I sincerely doubt that sufficient financing has been incorporated into the final version of the bill.”

Under the GOP proposal, states would get $100 billion over a decade to stabilize insurance markets or fund high-risk pools, but experts worry that may not be enough and will eventually force states to pick up extra costs.

“Those pools would be very important to people with serious mental illnesses ... and $10 billion a year may seem like a lot, but will offset only a fraction of the actual deficit,” Paul Gionfriddo, president and CEO of Mental Health America, noted in a blog posted Wednesday.

The House bill leaves in place protections for people with pre-existing conditions including mental illness, and it keeps in place the essential health benefits requirements for individual plans. Though it does not address mental health parity per se, Rep. Tim Murphy (R-Pa.) plans to offer an amendment during the Energy and Commerce Committee markup that would codify federal parity protections for people seeking mental health and substance abuse treatment.